Among the polyalkylene oxides, a water-soluble polymer polyethylene oxide has been used in diversified applications, such as thickeners for paper making, ceramic binders, aids in suspension polymerization, and raw materials for pharmaceutical formulations.
This polyethylene oxide is a linear polymer, so that much of the polymer physical properties are dominated by its molecular weight. Therefore, commercially available polyethylene oxides are mostly classified into grades according to their molecular weights.
The molecular weight of the polyethylene oxide is generally within the range of from 100,000 to 10,000,000 or so, and it is considered to be difficult to economically produce polyethylene oxides having molecular weights of from 100,000 to 2,500,000 or so out of those polyethylene oxides by direct polymerization in a high yield. As a method for obtaining a polyethylene oxide having a molecular weight of from 100,000 to 2,500,000 or so, a method including degrading a polyethylene oxide having a molecular weight of 3,000,000 or more produced by polymerization by gamma-ray irradiation has been generally known. However, the gamma-ray irradiation can only be applied in a publicly approved facility, in a limited area, so that there are some disadvantages in transportation costs and complicatedness of quality control. Further, the polyethylene oxide after the gamma-ray irradiation has some disadvantages of being poor in storage stability, lowering its molecular weight with the passage of time.
In view of the above, various studies have been made on molecular weight control with means other than the gamma-ray irradiation. As the degradation treatment other than the gamma-ray irradiation, the following methods have been reported.
For example, in Patent Publication 1, a peroxide is added to a polyethylene oxide, and the mixture is then heat-treated, thereby lowering an aqueous solution viscosity of a polyethylene oxide. In addition, in Patent Publication 2 it is mentioned that heat treatment is carried out under an oxygen concentration of from 10 to 500 ppm in addition to the same conditions as above, whereby the amount of a peroxide or a radical initiator can be reduced to from 0.5 to 5% by mass of the amount, based on the polyethylene oxide. In Patent Publications 3 and 4, a silicon dioxide powder is used as an anti-caking agent together with a polyethylene oxide, an aliphatic hydrocarbon, and a radical initiator.